When human erythrocytes (RBC) in vitro interact with natural tetrapyrroles such as bilirubin and protoporphyrin IX (proto), they become sensitized to hemolysis in visible light. Although the photo-lesions which predispose the cell to lysis arise within the plasma membrane, little is known about the molecular basis of their formation. Recent studies in this laboratory have shown that irradiation of RBC in the presence of proto, or isolated RBC membranes in the presence of proto or bilirubin, causes lipid peroxidation and cross-linking of major membrane proteins such as spectrin. The latter is observed by SDS-gel electrophoresis (DSD=PAGE) and by SDS-chromatography on Agarose columns. We speculate that polypeptide cross-linking in terms of reaction mechanism (e.g. whether singlet O2 might be involved), and types of bonds formed (e.g. whether phospholipid free radicals might serve as cross-linking groups). SDS-PAGE has revealed that irradiation or proto-containing RBC also produces relatively low MW peptide photoproducts (e.g. a prominent band at approximately 29 kdaltons), which we now know to be polymers of hemoglobin (Hb) chains. This reaction (also demonstrated in purified Hb alone) will be examined more extensively as a simple analog of cross-linking among membrane proteins. BIBLIOGRAPHIC REFERENCES: Glyceraldehyde 3-Phosphate Dehydrogenase in the Isolated Human Erythrocyte Membrane: Selective Displacemnt by Bilirubin", Girotti, A.W., Arch. Biochem. Biophys. 173, 210 (1976). "Bilirubin-Sensitized Photoinactivation of Enzymes in the Isolated Membrane of the Human Erythrocyte", Girotti, A.W., Photochem. Photobiol. 24, 525 (1976).